The present invention relates generally to the field of spindle motors for disc drive data storage devices and, more particularly, to an "ironless" spindle motor.
Disc drive data storage devices, known as "Winchester" type disc drives, are well-known in the industry. In a Winchester disc drive, digital data is written to and read from a thin layer of magnetizable material on the surface of rotating discs. Write and read operations are performed through a transducer which is carried in a slider body. The slider and transducer are sometimes collectively referred to as a head, and typically a single head is associated with each disc surface. The heads are selectively moved under the control of electronic circuitry to any one of a plurality of circular, concentric data tracks on the disc surface by an actuator device. Each slider body includes a self-acting hydrodynamic air bearing surface. As the disc rotates, the disc drags air beneath the air bearing surface, which develops a lifting force that causes the slider to lift and fly several microinches above the disc surface.
In the current generation of disc drive products, the most commonly used type of actuator is a rotary moving coil actuator. The discs themselves are typically mounted in a "stack" on the hub structure of a brushless DC spindle motor. The rotational speed of the spindle motor is precisely controlled by motor drive circuitry which controls both the timing and the power of commutation signals directed to the stator windings of the motor.
The first Winchester disc drives to be produced were large cabinet models which included discs having a diameter of 14 inches and AC induction spindle motors. These types of disc drives were commonly located in dedicated "computer rooms" with large mainframe computers, where environmental factors such as temperature and humidity could be carefully controlled. In this type of environment, the acoustic noise generated by cooling fans and disc drive motors was of little concern, since the only persons directly in contact with the systems were maintenance personnel, who were generally not in the computer rooms for extended periods of time. The users of such systems were typically located at a remote location and communicated with the computer system via keyboards and display terminals which did not generate excessive amounts of acoustic noise.
More recently, personal computers have become more popular and are commonly located within the work space of the system user. This has prompted an increase in awareness of acoustic noise generated by the disc drives located within the personal computers. In certain markets, such as Europe, the amount of acoustic noise allowable in the work place is closely regulated. With this in mind, it has become common for system manufacturers to impose a "noise budget" on manufacturers of major system components, such as disc drives, which limits the amount of acoustic noise that such components can contribute to the overall noise of the system.
One of the principal sources of noise in disc drive data storage devices is the spindle motor which drives the discs at a constant speed. Typical spindle motor speeds have been in the range of 3600 RPM. Current technology has increased spindle motor speeds to 4800 RPM, 7200 RPM and above. Analysis of various types of disc drives has brought to light several different modes of acoustic noise generation which are attributable to the spindle motor and its control logic.
One mode of noise generation is axial and radial vibration in the stator and rotor. The vibrations are caused by rotation of the rotor mass and by electromagnetic disturbances generated during excitation of the stator by the application and removal of commutation pulses. The commutation pulses are timed, polarization-selected DC current pulses which are directed to sequentially selected stator windings to drive the motor and control its speed. Rapid rise and fall times of these pulses act as a striking force and set up sympathetic vibrations in the stator and rotor which generate resonant vibrations in the housing causing unacceptable levels of acoustic noise.
Prior art attempts to reduce or eliminate noise include controlling the resonant frequency of the housing, damping the vibration of the housing, resin bonding the stator to the base, and centering the stator axially and radially with respect to the rotor magnet. In U.S. Pat. No. 5,376,850, acoustic noise is reduced by uncoupling the stator from hard contact with the stationary portion of the shaft.
Disc drive manufacturers have recently begun looking at replacing conventional ball bearings in spindle motors with hydrodynamic bearings. Hydrodynamic bearings are much quieter and in general haver lower vibrations than conventional ball bearings. However, more complete solutions are desired to further reduce acoustic vibration levels in disc drive spindle motors.